Numerous applications requiring storage and later retrieval of digitized speech signals exist. Audio response units which store prerecorded messages or vocabularies of instruction words and/or speech filing systems that record digitized versions of on-line analog speech messages for storage and later retrieval for a telephone answering application exist. With increasing emphasis placed upon office communications for the future, speech digitization and filing for later retrieval under computer control is an increasingly important application for this technology.
Typical prior art speech filing systems take analog speech signals from a telephone line, convert them to a low rate digital code and store the results in a digital storage medium such as a magnetic disk. Parallel analog tone control of the system is usually provided using a conventional telephone tone signaling key pad. To conserve memory space, low rate speech coding techniques, of which several are available, are employed. Disk storage requirements are further reduced by removing the silence gaps inherent in most speech input signals prior to the storage of the actual signals received. The gap information is traditionally retained in a run length encoding technique which is stored, in multiplexed fashion, with the digital information of the coded speech signal on the storage medium. During playback, the gaps are reinserted in the speech message stream to achieve a natural sound for the listener.
A problem with the known prior art is that a conventional hybrid circuit converts the two-wire telephone line to the four-wire speech encoder and decoder interface. This places severe demands and conflicting requirements upon the parallel tone receiver employed. During speech input, the receiver input will consist both of the remote control signals in the form of parallel encoded tones to which the receiver must react and to the analog speech input signals which must be recorded and to which it should not react. The receiver has to have a good "talk off" protection circuit so that the receiver will not falsely interpret portions of the speech signal spectrum as control tone pairs. Most competitive high performance parallel tone receivers have this facility. However, during playback of the stored speech, inputs to a parallel tone receiver would ideally be only the remote control parallel tone signals if the hybrid circuit were a perfect one.
Unfortunately, no perfect hybrid circuits exist and there is a significant loss or leakage between the legs of the hybrid circuit as will be readily understood by those of skill in the art. Typically, the losses from the transmit to the receive leg are in the range of 15db. Given a playback transmit level of 0dbm, a leakage signal will be present at the input leg of the hybrid circuit of -15dbm or more. Typical input signal levels from the analog telephone line are in the range of -25dbm. Therefore, during playback mode, incoming control signals at -25dbm will simply be swamped by the -15dbm leakage signal from the playback of previously recorded messages being sent out through the hybrid circuit. The normal "talk off" circuitry of the receiver will completely prevent the control signals from being received during this phase of operation. The only opportunity for response would be during the inserted gap periods when the playback signal level is near 0. When the gap is long enough, the receiver could respond to control signals but the automatic gain control circuit normally incorporated does not allow a fast recovery and the receiver will not respond to control signals during the gap time for short gaps.
Thus, for the duration of playback, the user at the remote end will be unable to signal control information to the system from his end of the line. The problem is real and has been demonstrated in existing equipment.